1. Field of the invention
This invention relates to an inkjet printing apparatus which ejects a liquid toward a print medium to perform print.
2. Description of the Related Art
There is known an inkjet printing apparatus which ejects ink toward a print medium from a printing head to perform print. In such an inkjet printing apparatus, in general a downsized printing head in which a plurality of nozzles ejecting the ink are formed in high concentration is used to perform high-fineness print. Further, a plurality of these downsized printing heads are located to supply ink of different colors to the respective printing heads, thereby making it possible to perform color print onto a print medium with a relatively inexpensive and downsized construction. Therefore, the inkjet printing apparatus is used in various printing apparatuses such as a printer, a facsimile and a copier whether it is for business use or for household use.
In such an inkjet printing apparatus, it is important to maintain ink in the printing head to be in a predetermined negative pressure (maintain a pressure exerting on the ink in the printing head to be in a predetermined negative pressure) for stabilizing an ink ejection operation from the printing head. Therefore, a negative pressure generating device is generally provided in an ink supplying system supplying ink to the printing head and the ink to which the negative pressure is applied by the negative pressure generating device is supplied to the printing head.
Japanese Patent Laid-Open No. 2002-1988 discloses, as the negative pressure generating device, the construction of generating a negative pressure by using a capillary function of a sponge-shaped ink absorber accommodated in an ink tank. Further, Japanese Patent Laid-Open No. 06-198904 discloses, as another negative pressure generating device, the construction provided with a flexible ink bag and an arched spring. In addition, Japanese Patent Laid-Open No. 2003-11380 discloses, as the other negative pressure generating device, the construction where an ink tank is located at a position lower than a printing head and a negative pressure is applied to ink by using a water head difference between the printing head and the ink tank.
In the ink supplying system equipped with the negative pressure generating device as disclosed respectively in Japanese Patent Laid-Open No. 2002-1988 to Japanese Patent Laid-Open No. 2003-11380, the negative pressure in the printing head increases with the ink ejection from the printing head. The ink is supplied from the ink tank to the printing head by taking advantage of this increasing negative pressure. Therefore, when a great amount of the ink is ejected per unit time from the printing head, the ink supply from the ink tank to the printing head does not possibly match the ink ejection amount. Therefore, the negative pressure in the printing head may be larger than a predetermined negative pressure. In reverse, when a small amount of the ink is ejected per unit time from the printing head, the negative pressure in the printing head may be smaller than the predetermined negative pressure due to inertia of the ink.
For solving such an issue, Japanese Patent Laid-Open No. 2006-326855 discloses the construction where ink supply to a printing head is carried out by a pump and a negative pressure in the printing head is controlled by a fan, thus carrying out the supply of the ink and the control of the negative pressure separately.
However, in a case of directly controlling the negative pressure in the printing head by the fan, it is required for the negative pressure control to respond quickly to a pressure fluctuation in the printing head. That is, since the negative pressure generated by the fan acts directly on an inside of a nozzle, it is required that the negative pressure control responds to a pressure in the printing head changing with an ejection state of the ink to immediately carry out the follow-up to the pressure fluctuation. Conventionally, the rotational speed of the fan is kept constant and in such a state, the follow-up is carried out by moving air in response to a pressure difference change between a suction port and a discharge port of the fan or in a case where the pressure change is large for a short period of time, it is required to control the rotational speed of the fan.
Conventionally, a range in which the pressure fluctuation in the printing head can be absorbed in a state of maintaining the rotational speed of the fan to be constant, is limited. In a case of controlling the rotational speed of the fan, it is required to control the fan in high responsiveness for maintaining the negative pressure in the printing head to be constant and further, it is required to control the rotational speed of the fan even in consideration of responsiveness of the pressure change in the printing head at the time of changing the rotational speed of the fan. In consequence, it is unavoidable for the control of the fan to be complicated.
In addition, in a case of controlling the negative pressure in the printing head by the fan, the ink in the printing head is directly stirred by the fan. Therefore, evaporation of water components contained in the ink is promoted, thereby possibly increasing viscosity of the ink. In a case where the ink exchange becomes necessary due to degradation by the increased viscosity of the ink, new ink is required, possibly increasing the running cost.